A battery is an electric cell or a device that generates electricity from a chemical reaction. It converts chemical energy stored into electrical energy by redox (reduc- tion/oxidation) reactions. Redox reactions are chemical reactions in which electrons are produced by the chemical reaction. It includes two or more cells connected in parallel or series. A cell consists of a negative electrode, positive electrode, a sep- arator, an electrolyte which conducts ions and an ion conductor. The electrolyte used in the battery can be aqueous or non aqueous in solid, liquid or paste form.
If the external load is disconnected from the cell, the reaction process stops, when the external load connected to the cell, the negative electrode supplies a current of electrons that flow through the load and are accepted by the positive electrode. The main parameters that predict most of the basic characteristics of the battery are the materials employed for the electrode and electrolyte for both the reduction and oxidation reactions. The electrode is the main location where core of the reduction and oxidation reactions take place. Most of the battery systems, containing alkaline and lead acid batteries, the electrode is not only where the electron transfer takes places, but also a component in the chemical reaction that generates or uses the electron. In some other kind of battery systems the electrode material is itself inert i.e., no reaction takes place at the electrode and it is the only place for the electron transfer from one reactant to another, this phenomena can be observed in fuel cells.
1.1 Batteries 3 During discharging process oxidation reaction occurs at the positive electrode called anode and it has positive voltage, reduction reaction during charging process occurs at the negative electrode called cathode, which is having negative voltage. While recharging process the cathode has a positive charge and the anode has a negative charge. The redox reaction occurs not only in the porous electrode, because a redox reaction includes the interaction of more than one component. The other compo- nent involving chemical reactions is the electrolyte. Most of the battery system, the electrolyte is an aqueous solution. Main reason for having an aqueous solution is the reduced or oxidized form of the electrode which exists in an aqueous solution.
Also another reason is that the chemical species in the electrolyte can flow freely inside the porous electrode where the chemical reaction occurs and ion species can move from one electrode to the other.
1.1.1 Types of Batteries
Iron Chromium Zinc cerium Zinc bromine Polysulphide bromide Soluble lead acid
All−vanadium Primary Batteries
Flow Batteries Battery systems
Secondary Batteries
Redox flow batteries
Vanadium bromine
Figure 1.1: Flow chart shows types of battery system
There are two types of batteries, primary and secondary batteries. In primary battery electrical energy conversion from chemical energy is one way process. Even though there are those types of batteries, which transforms irreversibly from chemi- cal energy to electrical energy, if the initial supply of reactants, is exhausted, energy cannot be restored to the battery. Therefore, the process is irreversible and electri-
cal energy cannot be converted to chemical energy. It is understood that a primary battery cannot be recharged, it will convert its chemicals into electricity one time only, then it should be discarded. There are few types of primary batteries such as, telegraph circuits, which can be restored to operation by replacing few compo- nents of the battery consumed by the chemical reaction. Figure 1.1 shows types of battery systems. In secondary batteries energy conversion process is reversible, it means that chemical energy can be convert into electrical energy and vice versa.
Also these batteries are rechargeable. Battery chemical reactions can make reversed by supplying the electrical energy to the cell, therefore it can restore to the original condition. In secondary batteries the electrodes can be reconstituted by transfer- ring electricity back through it, hence it is called a rechargeable or storage battery.
Examples of secondary batteries are lithium-ion batteries employed in higher power consumer electronic equipment such as camcorders, computer laptops, some digital cameras and mobile telephones and lead acid batteries used in cars. Silver zinc battery developed in 1960’s was one of the first space battery dominates in most of the industries. This battery has high specific energy and power but it is little costly due to the use of silver. Nowadays these batteries are still used in few s.elected applications, such as torpedoes and space launch vehicles. Silver zinc battery also employed in Mars pathfinder, but it was designed as a rechargeable battery. This type of battery has a relatively short cycle life and can not be used for multi year missions. This battery are also commonly used in hearing aid applications.
1.1.2 Flow Batteries
Flow batteries are also known as redox flow batteries. Redox stands for reduc- tion/oxidation reaction. The name redox flow battery is based on the redox reaction between the two electrolytes in the system. The chemical reactions encompass all chemical processes in which atoms have their oxidation number changed. A flow battery is a form of battery in which electrolyte consisting one or more dissolved electroactive species flows through a cell or porous electrodes and the reactor in which chemical energy is converted to electrical energy. Generally the electrolytes are stored externally by reservoirs/tanks and pumped through the cell of the reac- tor. A flow battery is designed in such a way that it is more flexible, because energy and power capacity of the cell can be changed very easily. Also the flow battery can be designed for high capacity electrical energy storage as well as for high power
1.1 Batteries 5 applications. There are many kinds of electrical energy storage devices, such as flywheels and conventional batteries that may not show the flexibility and have few limitations. Flywheels are generally employed for short durations of less than 300 seconds and high power storage more than 0.5×106 W, and conventional batteries are used for power less than 0.5×106 W and durations more than 1 hour. Large scale projects need high power storage and capacity storage, so preferably flow batteries are used. Therefore, flow batteries are used in the application of grid-connected elec- tricity storage at solar photovoltaic systems and wind farms. There are many kinds of flow batteries using aqueous electrolytes. These include polysulphide bromine, iron chromium cell, all-vanadium redox flow cell and zinc bromine cell. All flow cells use different electrolytes in the negative and positive half cells but cells have same design. Each cell is different in terms of energy efficiency, coulombic efficiency, cycle life and open circuit potential but cells face similar challenges such as electrolyte management and membrane failure. Flow battery is charged and discharged by a reversible chemical reaction process between the two liquid electrolytes of the bat- tery. These electrolytes are stored in the separate storage tanks, but in the case of conventional battery the electrolytes are stored in the power cell of the battery.
While operation the electrolytes are supplied from tank using pump through the electrochemical reactor in which chemical reduction and oxidation reaction occur and electricity is generated. Electrolytes are stored outside the reactor, so the de- sign specifications of the cells are flexible, therefore energy content and power of the system can be described separately. It is easy to replace the electrolytes or to increase the amount of electrolytes. Furthermore, the design of the power can be optimized for the power required, this is independent of the amount of electrolyte used. Flow batteries can be more attractive for future applications, particularly for large-scale applications, like peak power support at wind farms or solar photovoltaic cells.
1.1.3 Redox flow battery
A redox flow battery (RFB) is an electrochemical system that permits energy to be stored in two solutions encompassing different redox couples. RFBs are stationary storage batteries that operate continuously pumping two electrolytes through a pair of high surface area electrodes that are separated by the ion exchange membrane.
The energy is harvested and stored by the reduction/oxidation reactions of redox
active solutes in the two electrolytes. The redox flow battery system capacity is in- creased by simply increasing the volume of the electrolyte reservoirs or increasing the concentration of the electrolytes but the conventional secondary batteries storage capacity depends on the size of the electrodes. The redox flow battery technology is developing and having future, three alternative systems have recently been com- mercialized like, zinc-cerium system, polysulphide-bromine system and all-vanadium system. Figure 1.2 describes redox flow battery which consists of a battery cell and
Figure 1.2: Schematics of the vanadium redox flow battery
storage tanks for positive and negative electrolytes. The battery cell is separated by a membrane to provide positive and negative cell, each cell having a liquid per- meable porous electrode [3, 4, 5, 6, 7, 8]. The electrolyte is prepared by mixing the electroactive materials with sulphuric acid aqueous solutions. The positive and negative electrolytes are circulated from the storage tanks to the positive and nega- tive cells, respectively [3]. In the battery the reduction and oxidation reactions take place on both sides of the membrane. While the discharge, electrons are removed from the anolyte and passed through the external circuit to the catholyte. While charging flow direction of electron is reversed, the reduction occurs in the anolyte and the oxidation process takes place in the catholyte [9].